Multi-phase or multi-component (from herein called multiphase) flow is widespread in many engineering disciplines. In some cases, the geometry which defines the boundaries confining the multiphase flow are known precisely, such as a system of pipes or channels. Other scenarios involve highly random and complex geometry, such as the pore space of a porous rock. One example of an application of multiphase flow through a complex porous media is the study of oil and water flow in oil reservoir rock. Another example is oil, water, and gas in reservoir rock. Another example is liquid and gas flow in a porous material acting as a gas diffusion layer in a chemical separation process. A key property of this flow behavior is the “relative permeability” of each fluid phase, which characterizes how easily that phase moves through the rock for the specified conditions. In multiphase flow in porous media, the relative permeability of a phase is a dimensionless measure of the effective permeability of that phase. It is the ratio of the effective permeability of that phase to the absolute permeability.
An especially important aspect of the relative permeability of oil/water flow in porous rock is the behavior at and near the “residual oil saturation”. The term saturation refers to the volume fraction of water over total fluid (oil and water), and residual oil saturation refers to the saturation value for which the average oil velocity (and hence the oil relative permeability) is essentially zero. Since for this condition the oil cannot move, the corresponding volume fraction of oil is known as the “residual oil”. This is an important quantity because it represents the minimum amount of oil that cannot be extracted from the rock, hence the residual oil saturation value, and the relative permeability as a function of saturation as the saturation approaches the residual oil saturation value, is especially valuable information for determining the value of the reservoir and for assessing potential enhanced oil recovery (EOR) methods.
For multiphase flow measurement or prediction methods, it is therefore important to have a highly accurate method to determine the permeability of the fluid in the porous material even when it is very tiny, since a key goal is to find the saturation value for which it effectively reaches zero.